The maximum quantity allowed for purchase is 0.

nopCommerce version  4.20
Operating system  Microsoft Windows NT 6.2.9200.0
ASP.NET info  v4.0.30319
No idea on theme?

Hi Andy007,
I'll request you to go through some dotnet core tutorials first. Then start nopCommerce.

Still does not work

So you have not installed an extra (non-standard) plugins
You are using the Standard Theme - Default clean
See http://yourwebsite.com/Admin/Setting/GeneralCommon

When you goto the product page - Check if there are any script errors
i.e. in Chrome - Right Mouce Click > Inspect

Sorry Duplicate post - See above